It has become common place to find receiver and transmitter circuits in a radio communication unit employing digital mixers instead of conventional analog mixers. Digital mixers perform substantially the same tasks as analog mixers. For example, for transmitter circuits, digital mixers can be used to up-convert a baseband signal to an IF (intermediate frequency) signal, where as for receiver circuits, digital mixers can be used to down-convert a high frequency signal to an IF signal. These are conventional operations of analog mixers. The rationale for changing to digital technology is based on the substantial benefits attained over analog technology in areas such as manufacture, quality, power consumption, and flexibility for redesign and control.
FIG. 1 illustrates a prior art structure for down-conversion digital mixers 100. The digital mixer 100 includes an ADC (analog-to-digital converter) 104, a digital filter 110 and a mixing element 114. Alternatively, an up-conversion digital mixer 100 is employed with an analog-to-digital converter at the output of the mixing element 114. A popular type of ADC is a sigma-delta converter. This type of ADC is popular because it allows for digital sampling of high frequency RF signals. Some structures have been known to sample RF signals operating at a carrier frequency of 600 MHz.
In order to perform the conversion of an analog signal 102 to a digital RF signal 108 operating at the same carrier frequency (e.g., IF.sub.1), a sampling clock 106 is provided to the ADC 104. This sampling clock 106 is selected in accordance with the operational characteristics of the ADC 104. Independent of the carrier frequency of the input signal 102, the sampling clock 106 must operate at a fixed frequency, and cannot be arbitrarily varied. The inflexibility of varying the sampling clock 106 is a crucial limitation of prior art systems, for reasons that will become apparent shortly.
The digital filter 110 is generally used as an anti-aliasing filter as well as a pre-selection filter for filtering out unwanted frequencies in the digital signal 108. The filtered digital signal 112 generated therefrom is applied to the mixing element 114. The mixing element 114 is a digital mixer controlled by an injection signal 116 operating at a mixing frequency. The injection signal 116 conforms to the equation: ##EQU1##
Mathematically, the mixing operation is simply the complex multiplication of the filtered digital signal 112 and the injection signal 116. Because digital multiplication operations consume a high amount of power, an undesirable factor in low-power consumer products, and because multiplication operations at high frequencies (e.g., 600 MHz) is not always achievable, the simplification of the injection signal equation is paramount. To eliminate the need for complex multiplication, prior art systems set the mixing frequency f.sub.mix to one-quarter the sampling frequency f.sub.s. By doing this the injection signal can be simplified to the equation (j).sup.n. This equation results in a simplified periodic series of constants (1,j,-1,-j,1,j-1,-j, . . . ).
Utilizing a much simpler injection signal such as (j).sup.n completely eliminates the need for a digital multiplier unit. Instead, a conventional digital logic can be used, which converts the digital values provided by the filtered digital signal 112 to complex in-phase and quadrature digital signals 118 without utilizing a digital multiplication circuit.
The elimination of high power consuming digital multipliers has proven to be a very effective means for utilizing digital mixers 100 in low power selective call units. However, the simplification method presented above, results in a significant constraint in the design of digital mixers. That is, this method provides for only one frequency conversion. Assuming the digital signal operates at an IF frequency IF.sub.1, the frequency conversion to a new IF frequency IF.sub.2 follows the equation: ##EQU2##
As should be apparent, the conversion process is limited to one mixing frequency (or injection frequency) f.sub.s /4. Restricting, the mixing frequency f.sub.mix to a single injection frequency, eliminates the flexibility of mixing a digital signal to any arbitrary frequency (e.g., baseband). This is a severe limitation when contrasted to analogous analog mixers.
Thus, low-power prior art digital mixers, employing the above method, are substantially limited by the sampling frequency 106 used by the selected ADC 104. Accordingly, it is desirable to provide an improved digital mixer for selective call units that is capable of mixing digital signals at an arbitrary mixing frequency f.sub.mix. Moreover, the improved digital mixer should be capable of performing either up or down frequency conversion.